Add boot-once support

This allows to specify an exceptional boot order only for the first
startup of the guest. After reboot, qemu will switch back to the default
order (or what was specified via 'order='). Makes installing from CD
images and then booting the freshly set up harddisk more handy.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>